In view of the ever-stricter safety regulations concerning lamps and in particular gas discharge lamps, which are intended for open lighting units, i.e. units without any protective glass in front of the lamp, the requirement exists that throughout the life of such lamps a completely safe and reliable operation is ensured. In the case of such lamps the very limited risk exists that the gas discharge tube present in the gas discharge lamp will burst during operation. In such a case glass splinters could shatter the surrounding gas discharge lamp bulb and possibly endanger persons or materials.
It must therefore be ensured with such gas discharge lamps, that the outer bulb thereof does not break. However, even in the case of lamps having protective glass there is a very limited risk that splinters emanating from the lamp will shatter said protective glass.
A lamp of the aforementioned type, whose design already partly takes account of problems of the aforementioned type is known from U.S. Pat. No. 4,721,876. The gas discharge lamp described in this document has an inner envelope bulb, which is designed as a cylindrical bulb open on two sides or as a dome-like bulb open on one side and having a relatively thick cylindrical wall. This known envelope bulb is surrounded by a wire gauze or grid. The gas discharge tube, including the envelope bulb and wire gauze, are encapsulated in vacuum-tight manner with respect to the outside in a glass bulb.
Apart from the fact that this solution is unfavorable for various metal vapour lamps, because the wire gauze can influence the electrical potential of the lamp and can therefore lead to an increased sodium loss and therefore to a shorter life, the envelope bulb is open on at least one side. This leads to the high risk of splinters passing through the open end of the envelope bulb in the case of the gas discharge tube bursting and might even shatter the outer bulb. In this known lamp there is also no UV-protection.
A lamp comparable with that described hereinbefore is known from EP 361 530 A or U.S. Pat. No. 4,942,330. For protection against bursting or shattering of the gas discharge tube said lamp has a transparent plate, which is constructed as a cylinder open on both ends and which is surrounded by a net of ceramic fibres. These ceramic fibres are intended to have an adequate strength to ensure that if a gas discharge device bursts, splinters can be held back if the plate breaks.
Apart from the fact that the plate is open at both ends, a serious disadvantage is represented by the fact that the ceramic net is difficult to manufacture and difficult to place around the plate. Moreover, the plate, in conjunction with the ceramic net, reduces the light flux of the lamp.
In a gas discharge lamp according to EP 186 899 B1 , the bursting protection is constituted by a cylindrical element in the form of a quartz sleeve. An outer holding wire is provided for the precise positioning thereof around the gas discharge tube, but no shattering protection is provided.
Further quartz bulbs closed on one side and placed around the gas discharge tube are known from EP 173 235 B1or 165 587 B1. The envelope bulbs open at one side provided therein are primarily used for the thermal operating conditions of the lamp, because the open end of the envelope bulb cannot offer a protection against splintering with respect to the outside and there is no other splintering protection means around the envelope bulb.
Other measures intended to ensure increased safety in the case of gas discharge lamps are cylindrical envelope bulbs with a wall thickness of 2 mm and more, but these are at least open on one side and usually on both sides. A further possibility are two telescoped quartz glass protective tubes, which surround the gas discharge tube, a UV-filter being applied to one protective tube.
Another problem with gas discharge lamps is the necessary UV-protection which such lamps must have, i.e. the emitted radiation in the UVC, UVB and UVA ranges must be at least limited to specific, prescribed values.
The known gas discharge lamps use UV-filters applied by sputtering or dipping. This application of the UV-filter normally takes place to the inner envelope bulb or to the outer glass bulb. However, these solutions suffer from the disadvantage that such applied filters undergo changes to their characteristics during the life of the lamps. In addition, the transmission characteristics of these UV-filters are highly dependent on the light incidence angle and the temperature of the filter coating applied.